A multi-stage flash evaporator is the main component of a seawater desalination plant for producing distilled water from seawater. Most evaporators for large capacity desalination plants are currently of a ‘cross tube’ type with all flash stages arranged in a single tier configuration, being build with evaporator unit capacities up to about 25 million gallons per day or about 100,000 cubic meters per day of distillate production.
A multi-stage flash evaporator comprises a plurality of flash stages, in existing plants typically between 15 and 30. While a heated solution, typically seawater or brine, enters the first flash stage at its highest temperature, the solution flashes down in each consecutive flash stage to a lower temperature compared to the temperature of the solution in the previous flash stage, releases some vapor which is then condensed on a tube bundle and collected as distillate. The salt concentration of the solution is increasing toward the last flash stage. A coolant enters with its lowest temperature into the tube bundle(s) at the last flash stage and passes through all tube bundles toward the first flash stage, while its temperature increases in each flash stage relative to its temperature in the previous flash stage as vapor is condensing on the tube bundles. The coolant discharging from the tube bundle(s) of the first flash stage is further heated in a separate heat exchanger, commonly described as the heat input section or brine heater, by an external heat source to a top temperature. The coolant is then used as the solution, also described as flashing brine, fed into the first flash stage.
A multi-stage flash desalination system may be designed as a “once through” process in which one type of coolant is being conveyed through the tube bundles of all flash stages, starting from the last flash stage with the lowest operation temperature to the first flash stage operating at the highest temperature.
The most common design concept for multi stage flash desalination plants is the “brine re-circulation” system, in which the evaporator comprises a heat recovery section and a heat rejection section. The heat rejection section comprises a plurality of flash stages including the last flash stage, in which typically fresh seawater is used as a first coolant for the tube bundles. The heat rejection section is designed such, that the first coolant is capable to remove together with the discharging distillate and the discharging concentrated solution, the majority of the heat introduced into the system through the heat input section. In the heat recovery section, which occupies typically the larger number of flash stages of an evaporator including the first flash stage, the heat released from the solution is recovered by a second coolant and used to bring the second coolant toward the desired top temperature. A mixture of a part of the concentrated solution discharging from the last flash stage of the evaporator and a part of the first coolant discharging from the heat rejection section, described mostly as re-circulating brine, is commonly used as the second coolant for the heat recovery section. The portion of the first coolant used as part of the second coolant, replaces primarily the amount of distillate and concentrated solution discharging from the system. It may be treated in order to limit scaling of the tube bundles and to limit corrosion in the evaporator.
Individual types of evaporators may be differentiated by the tube bundle configuration such as ‘long tube’ evaporators and ‘cross tube’ evaporators. In a long tube evaporator, the tube bundles are substantially oriented in the flow direction of the solution in the flash stages. A long tube evaporator typically comprises a plurality of individual evaporator modules. Each module comprises typically one tube bundle with a tube sheet and a water box on each end. The individual evaporator modules are typically internally divided by partition walls into a plurality of flash stages. The tube bundles are also divided by the partition walls into a plurality of tube bundle elements, so that each flash stage comprises one tube bundle element, configured to condense the vapor released from the solution in the individual flash stages. The coolant is typically conveyed through the tube bundles of the individual modules of an evaporator unit in serial flow communication. Evaporator modules comprising two tube bundles fed with coolant in parallel have been designed and build as well. Long tube evaporators have been preferred until about the early 1980's when the maximum evaporator capacities have been in the range of about 30% of current evaporator capacities. The modules of a long tube evaporator may be also arranged in a vertically stacked tier configuration, wherein each tier may comprise a plurality of parallel arranged tube bundles.
The cross tube evaporator became for larger capacities the preferred and more economical evaporator configuration. In a cross tube evaporator, the tube bundles are oriented substantially transversally to the flow direction of the solution in the flash stages. Cross tube type evaporators typically comprise an individual single pass tube bundle in each flash stage. Evaporator configurations with double pass tube bundles in each flash stage or common tube bundles for a pair of flash stages are also known. The cross tube evaporators of the prior art have technically limited possibilities to increase the unit capacities beyond the maximum unit capacities of evaporators currently in operation, mainly due to limitations of available tube length for tube bundles.
The cross tube type evaporators with large unit capacities are typically designed and built in a single tier configuration, meaning, all flash stages being arranged on the same level and operating in a serial flow communication. Multiple tier configurations, with flash stages arranged in two or more tiers stacked on top of each other, wherein each tier comprises a fraction of the total number of the flash stages and all flash stages operating in serial flow communication have been designed and built as well. In some stacked design configurations common horizontal tier partitions have been used between the tiers, while in other cases individual evaporator modules, each having its own shell roof and shell bottom structure, have been stacked on top of each other.